How High™ Electronic Altimeter Teardown Report

Winged Shadow Systems has developed a
solid state electronic altimeter that plugs into a spare receiver channel for power,
and provides altitude readings between 50 feet and 7,000 feet above ground level. My
How High™ is version 1 that was sold back in the 2006 - 2007
realm, so that is the one I will be describing here. There is a version 2 available now
with a couple additional functions, like storing the last altitude reading even after
power has been removed.

The heart of the system is the SM5420
pressure sensor, by Silicon Microstructures. It is a micromachined structure molded in
an 8-pin SOIC plastic package. Here is the
datasheet. Per the manufacturer, "The SM5420C is a small outline SO-8
packaged pressure sensor. The sensor uses SMI’s SM5108C micromachined, piezoresistive
pressure sensing chip that has been optimized to provide the highest possible accuracy
for a package of this size. This performance is achieved through careful resistor placement
and mechanical configuration along with advanced MEMS processing."

The SM5420 is fundamentally a
resistive bridge structure similar to the familiar Whetstone Bridge, as shown to the
left. So, in order for the How High™ to measure altitude,
it must obtain and store a reference value when the device is first powered on. Then,
an analog-to-digital converter feeds a custom programmed IC to store the highest level
recorded and display its content using a series of light pulses from an LED. A stable
voltage reference source helps ensure accuracy.

Looking at the circuit assembly to the left, the SM5420 package is in the upper right
corner. Interestingly, it has a U-shaped piece of magnet wire laying on top of the device
and soldered to the lead frame protrusions from the end of the molded plastic package.
My guess is that it either helps protect the circuitry from ESD discharges, or it somehow
helps prevent an electric field from accumulating around the micromachined structure
and disturbing the sensitivity of the measurements.

Surface mount packages are notoriously cryptically marked (due to not much room),
so I have to guess at some of the parts. In the lower right corner is a 5-pin SOT that
is likely the voltage reference.

In the upper left corner of the PCA (printed
circuit assembly) is a PIC12F683 8-bit microcontroller by Microchip Technologies. Here
is the datasheet. Since I do not see anything resembling a crystal, I assume
a resistor + capacitor is used for the clock generation. The datasheet allows for such
a scheme (see page 23). The PIC12F683 also contains an analog-to-digital converter (ADC)
and voltage comparator, but there is a separate, more precise IC used for that - the
Linear Technology LTC2480. The designer(s) at Winged Shadow Systems burns a custom program
into the PIC12F683 that performs all the nifty steps necessary to determine the maximum
altitude and to report the number to you, and even detects the wagging of your finer
in front of the LED to activate the report mode. A diode's transconductance is affected
by visible light impinging on its surface, so it is possible to use it as a light level
detector when housed in a transparent package, so it acts both as an LED for reporting,
and as a detector for initiating the report (very clever idea, I might add).

The Linear Technology
LTC2480 (in the center of the PCA) is a 16-Bit ΔΣ analog-to-digital converter (ADC) in
a 10-pin MSOP package (LBJY is the identifying marking on the package). Here is the
datasheet.
The printed circuit board traces run from the SM5420 to Pin 4 and Pin 5, which are the
Vin ports. The 4-wire SPI interface connects to the PIC12F683 controller.

I will not even attempt to guess at the details of programming because it is not important.
The important thing is that the designer(s) at Wing Shadow Systems are to be commended
for such a ingenious approach to this great little altimeter. Who would have imagined
such a precise device even a decade ago, before micromachining of silicon substrates
was able to be accomplished in productions quantities? They now also have a slick little
air speed indicator, but I do not have one of those. If somebody wants to send me one
(working or not), I'll be glad to do a reverse engineering report on it, too.

Even during the busiest times of my life I have endeavored to maintain some form
of model building activity. This site has been created to help me chronicle my journey
through a lifelong involvement in model aviation, which
all began in Mayo,
MD ...